For conventional image sensors including silicon substrates, noise in photodiodes and amplifier transistors formed on the same silicon substrate has been reduced, and a readout circuit has been improved to allow photon signals to be read out with low noise of 1 e−rms.
For a further reduction in noise in CMOS image sensors and a significant increase in dynamic range, not only noise in the photodiode, but also noise in a transistor forming a source-follower amplifier needs to be reduced. Successful photoelectric conversion of pixel signals with readout noise of less than approximately 15 e−rms enables achievement of an ultimate sensitivity with which a single photon signal is detected, that is, photon counting.